(1) Field of the Invention
The present invention relates to corrosion resistant agents and methods of making and using them to protect surfaces of metals that are subject to corrosion, and more particularly to corrosion-responsive agents that permit the use of reduced amounts of chromate or are free of chromate.
(2) Description of the Related Art
The protection of aluminum and aluminum alloys from corrosion is of wide interest, but in aircraft applications, it becomes critical. Aluminum alloys such as 2024 and 7075 are typical of the type used for aircraft service and these alloys characteristically contain copper. Although the presence of copper provides advantageous strength and other physical properties to the alloy, it nevertheless catalyzes the oxygen reduction reaction (ORR), which is a key element in corrosion processes.
The conventional method for protecting aircraft aluminum from corrosion involves the application of a conversion coating to the bare aluminum followed by applications of a primer and a topcoat. The topcoat provides the final color and surface texture and serves as a sealant for the undercoating. However, the conversion coating and the primer provide the majority of the corrosion resistance for the metal.
Chrome conversion coatings that contain hexavalent chromium are the present standard for use as conversion coatings for aluminum. Conventional hexavalent chrome conversion coatings meet Military Specification Mil-C-5541.
The present standard primer is a chromated epoxy primer meeting Military Specifications Mil-PRF-23377. Examples of this type of primer include Deft 02-Y-40A and Hentzen 16708TEP/16709CEH.
Typical topcoats for aircraft use meet Military Specification Mil-PRF-85285. Examples include Deft 03-GY-321 and Deft 99-GY-